Because of the ever-increasing cost of conventional energy sources such as oil, gas, coal, and electricity, there has been a corresponding rise in interest in less expensive energy alternatives. One such alternative is so-called "producer gas," a low Btu fuel gas whose oxidizable components comprise carbon monoxide, hydrogen and methane, the gas being obtainable from the partial combustion of waste carbonaceous materials such as wood chips, bark, sawdust, and other biomass sources such as ground corn cobs, lignite, peat moss, etc. However, a recurring problem in methods and apparatus for the production of such fuel gas is the generation of ash that tends to fuse into irregular-sized chunks, known as slag, the formation of which tends to block gas passageways and so reduce the efficiency of the pyrolysis of the solid waste materials. Another common problem which reduces pyrolysis efficiency is the buildup of condensates of tar and resin, resulting in blinding and otherwise restricting filters, grates, and gas passageways. Still another problem in the art is the production of an off gas from such solid waste pyrolysis that contains insufficient concentrations of combustible gases to comprise a useful fuel product. These and other problems are addressed and resolved by the pyrolysis reactors of the present invention, which are summarized and described in detail below.